1. Field of the Invention
The invention relates to an electrical load control device including an overcurrent abnormality determination device, an overcurrent abnormality determination method for an electrical load control device, and an overcurrent abnormality determination program.
2. Description of Related Art
Generally, a driving force distribution device of a four-wheel drive vehicle directly transmits a driving force from a transmission to either front wheels or rear wheels and also distributes and transmits the driving force to the other of the front wheels and the rear wheels through a variable torque clutch mechanism. By adjusting the transmission torque using this mechanism, the distribution of the driving force to the front and rear wheels is adjusted. As the variable torque clutch mechanism, an electromagnetic variable torque clutch mechanism is known. The electromagnetic variable torque clutch mechanism is controlled by a driving force distribution control device (ECU). That is, the frictional engagement force between clutch discs is variably controlled to control the fastening force by controlling the value of a current supplied to an electromagnetic solenoid using the ECU. Thus, the transmission torque is controlled. More specifically, the ECU generates a current command value (the amount of the current to be supplied to the electromagnetic solenoid), and calculates a duty ratio to eliminate the deviation between the current command value and the actual current (detection current) flowing through the electromagnetic solenoid by using proportional and integral control (PI control) and Pulse Width Modulation (PWM) control. The ECU controls the electromagnetic solenoid by outputting a control signal corresponding to the duty ratio to the driving circuit.
When a short-circuit failure occurs between both ends of the electromagnetic solenoid, there is a possibility that a hunting phenomenon may occur. The hunting phenomenon refers to a phenomenon in which excessive current (actual current) flows when a field effect transistor (FET) of the driving circuit is turned on in a state where a short-circuit failure occurs between both ends of the electromagnetic solenoid. Since the ECU performs the PI control, the ECU performs control so that the current is reduced to zero or a current value near zero by reducing the duty ratio when an excessive actual current is detected. When the current becomes zero, the ECU performs control so that a current flows again. As a result, an excessive current flows.
A driving force distribution control device that determines whether or not there is a short circuit between both ends of an electromagnetic solenoid based on the detection of a hunting phenomenon is known (for example, refer to Japanese Patent Application Publication No. 2004-142726 (JP 2004-142726 A)). In this driving force distribution control device, it is determined whether or not there is a short circuit between both ends of an electromagnetic solenoid based on the number of times of hunting of a current command value with respect to a threshold value within a time shorter than a predetermined time. When it is determined that there is the current command value, a microcomputer of an ECU accumulates the number of times of hunting with respect to the threshold value within a time shorter than the predetermined time, and determines that both ends of the electromagnetic solenoid are short-circuited if the number of times of hunting is equal to or larger than the predetermined number of times.
In the driving force distribution control device (ECU) described in Japanese Patent Application Publication No. 2004-142726 (JP 2004-142726 A), however, even when it is determined that there is the current command value, it is not determined that both ends of the electromagnetic solenoid are short-circuited unless the number of times of hunting reaches the predetermined number of times. While the number of times of hunting is accumulated, an overcurrent due to a short circuit between both terminals of the electrical load continues to flow to the electrical load control device. In addition, since a current flowing between a battery as a power supply and the electromagnetic solenoid is a pulsed current, a plurality of current A/D values sampled during a predetermined period are averaged. Therefore, when the electromagnetic solenoid is actually short-circuited, the value of an overcurrent is calculated to be low. As a result, there is a possibility that the abnormality cannot be detected.